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A026646 a(n) = Sum_{0<=i,j<=n} A026637(i,j). 1
1, 3, 7, 17, 37, 79, 163, 333, 673, 1355, 2719, 5449, 10909, 21831, 43675, 87365, 174745, 349507, 699031, 1398081, 2796181, 5592383, 11184787, 22369597, 44739217, 89478459, 178956943, 357913913, 715827853, 1431655735 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

a(n) indexes the corner blocks on the Jacobsthal spiral built from blocks of unit area (using J(1) and J(2) as the sides of the first block). - Paul Barry, Mar 06 2008

Partial sums of A026644, which are the row sums of A026637. - Paul Barry, Mar 06 2008

LINKS

Table of n, a(n) for n=0..29.

FORMULA

G.f.: [1-x^2+2x^3]/[(1-x)(1-2x)(1-x^2)]. - Ralf Stephan, Apr 30 2004

a(n)=J(n+3)-2*floor((n+2)/2), where J(n)=A001045(n) the Jacobsthal numbers; a(n)=1+sum{k=0..n, J(k+2)-1}; - Paul Barry, Mar 06 2008

a(n+1) = 2*a(n) + A109613(n), a(0)=1. - Paul Curtz, Sep 22 2019

MATHEMATICA

CoefficientList[Series[(1-x^2+2x^3)/((1-x)(1-2x)(1-x^2)), {x, 0, 29}], x] (* Metin Sariyar, Sep 22 2019 *)

CROSSREFS

Cf. A000126.

Cf. A026637, A026644, A109613.

Sequence in context: A244629 A065545 A110930 * A026668 A111210 A033489

Adjacent sequences:  A026643 A026644 A026645 * A026647 A026648 A026649

KEYWORD

nonn

AUTHOR

Clark Kimberling

STATUS

approved

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Last modified July 14 13:27 EDT 2020. Contains 335729 sequences. (Running on oeis4.)